The present invention pertains to the aircraft control art and, more particularly, to an aircraft lift control system with acceleration and attitude limiting.
The prior art has developed numerous aircraft lift control guidance systems. Such systems are useful during climbout operations, particularly in adverse environmental conditions such as windshear, as an aid to the flight crew in utilizing the aircraft's capabilities to optimize lift.
A particularly effective aircraft lift control system is shown in FIG. 1. While this system is described in detail in U.S. Pat. No. 4,326,253, issued Apr. 20, 1982 and assigned to the same assignee as the present invention, a brief explanation of system operation is given herein. Angle of attack sensor block 10 monitors aircraft angle of attack and produces an output signal .alpha. representative thereof. The angle of attack signal .alpha. is combined in a summer circuit 12 with the output signal, .alpha..sub.REF, from an optimum lift computer 14. Lift computer 14 receives an altitude rate signal, h, from the altitude rate block 16. Lift computer 14 responds to the h signal to produce a continuously variable .alpha..sub.REF signal between maximum and minimum limits.
The angle of attack signal .alpha. is subtracted from the .alpha..sub.REF, thereby producing an angle of attack error signal .alpha..sub..epsilon.. The angle of attack error signal is combined with rotation bias and path damping signals from block 18 in a summing circuit 20. As is well known in this art, the rotation bias and path damping signals are normally provided to initiate the climbout maneuver and to stabilize the system thereby preventing phugoid aircraft oscillation. The output from summer 20 is, thus, an angle of attack error signal with associated bias and damping signals.
A switch 22, as provided in the flight deck, is actuable to either a land or a go-around position depending upon the desired maneuver. In the land position, signals provided by the landing approach guidance block 24 are passed through a summer 26, which adds appropriate pitch rate signals from pitch rate block 28 thereby, as is generally understood in this art, avoiding additional sources of instability, and is selected as the elevator command signal .delta..sub.EC. If a go-around maneuver is desired, the switch 22 is thrown to its appropriate position whereby the angle of attack error signal with suitable damping signals is routed to the summer 26, with the combined signal being provided as the elevator command signal .delta..sub.EC. In this mode, any deviations of the aircraft's angle of attack from that as commanded by the optimum lift computer will result in an elevator command signal which tends to alter the aircraft's angle of attack until the desired optimum lift condition is realized.
The elevator command signal .delta..sub.EC is passed to the aircraft's flight control systems 29. Here, the .delta..sub.EC signal could be used to drive a flight director system pilot guidance display, thereby allowing the pilot to manually fly the aircraft in accordance with the .delta..sub.EC command, or the .delta..sub.EC signal could be directly coupled to the aircraft's control surfaces (such as the elevators) to automatically fly the aircraft in accordance with the .delta..sub.EC command.
While the system of FIG. 1 provides rate of climb and longitudinal acceleration as required to optimize the aircraft's climbout performance, it may result, at higher thrust to weight ratios, in accelerations which reach values of several knots per second. This rapid acceleration is undesirable to some aircraft operators since it tends to increase pilot work load due to the requirements of manual operation of the flaps to keep pace with the rapidly increasing speed of the aircraft at a time when pilot work pressure is at a peak, such as during a missed approach.
It is desirable, therefore, to provide a limit to longitudinal acceleration. Moreover, if, due to excess thrust and to limits imposed upon acceleration, an excessive pitch attitude develops, a means of limiting the pitch attitude to values compatible with passenger comfort may also be desired.